This invention relates generally to antennas for communication systems which incorporate means for automatic target tracking, and more particularly, to the improvement of a Cassegrain antenna utilized with such an automatic tracking communication system.
A typical five-horn Cassegrain antenna, as shown in FIG. 1, comprises a concave main dish 10, a convex subreflector 12 located near the focal point of the main dish 10, and a set of five radiation feed elements which are illustrated as the horns 14 and 16 located between the main dish 10 and the subreflector 12. The subreflector is operable to reflect energy between these horns and the main dish 10. In the transmit mode, the energy impinging on the maindish 10 from the subreflector 12 will be reflected out into the atmosphere in the form of an in-phase wave. The five radiation feed elements noted above comprise a sum feed represented by horn 14 centered on the boresight axis 15 of the main dish 10 and four error feeds represented by the horns 16 equally spaced around this sum horn.
When the energy is radiated by a transmitter a substantial distance out in space (essentially a point source), some of the radiated energy will be intercepted by the main dish 10 and rereflected toward the subreflector located at the focal point of the main dish. The subreflector 12 will, in turn, reflect this energy into the apertures of the five antenna horns 14 and 16.
The four error horn elements will receive target echo signals which have relative amplitudes which are proportional to the angular position of the target in a plane perpendicular to the boresight axis of the parabolic dish. If the target is directly on the boresight axis of the parabolic dish, then the signals received by the four outer horns will all have equal amplitudes and will be in phase. If, however, the target is off the boresight axis by a particular angle, then the signals received by the four outer horns will have different amplitudes by a predetermined amount. The signals intercepted by these error horns are paired and combined in the hybrid circuits 18 and 20 in the conventional manner to generate an elevation error signal and an azimuth error signal. The sum horn is utilized to obtain a sum signal which contains the data to be communicated, and to provide a phase reference for the error horn signals. If the system utilizes circular polarization, a conventional polarizer 22 may be utilized in the sum channel in conjunction with a conventional orthomode circuit 24 for providing either left or right circular polarization. (The sum horn is also used to transmit the radiation utilized by the main dish 10 to form the plane wave and to receive any information being transmitted by the target (a satellite)). The elevation error and the azimuth error signals may be switched and phase shifted and further combined with the sum signal to simulate a single signal being sequentially shifted about the boresight axis. The amplitude modulation of this combined signal may then be used to provide tracking error information to the tracking servo elements of a tracking receiver.
A historical problem in the design of a five-horn Cassegrain antenna has been the trade-off between a sufficiently large sum horn for good illumination efficiency and a sufficiently small error horn spacing for adequate error channel secondary pattern crossover. The utilization of a large diameter sum horn is generally required in order to obviate sum horn radiation spillover at the periphery of the subreflector. FIG. 2 shows a wide sum horn main lobe with its attendant spillover. According to standard antenna theory, the dimensions of a radiating antenna in the plane normal to the direction of transmission must be at least several wavelengths, and preferably more before significant directivity is achieved. Thus, in order to increase the directivity of the sum horn (decrease the width of the sum horn lobe) the sum horn diameter must be increased. By increasing the directivity of the sum horn lobe and thus reducing the spillover of the radiation at the subreflector, more energy is radiated by the main dish in the form of in-phase plane waves thereby increasing the efficiency of the antenna and permitting its use in long-range applications.
However, while large diameter sum horns are desired, it is also necessary to minimize the separation of the horn centers of the four surrounding horns in order to prevent the crossover of these outer horn radiation patterns on the first sidelobe or beyond. Such first sidelobe crossover is not desirable because of the sensitivity of the low level first sidelobes to various factors including reflector distortion, frequency change, and blockage. Moreover, because the first sidelobe normally has a low level, it is difficult to determine the precise position of the null in the crossover so as to generate the azimuth and elevation error signals. Therefore, prior art systems have generally tried to space the error horns so that the error horns secondary pattern crossover is on the main offset lobe or at the first null. The radiation pattern resulting from such a spacing is shown in FIG. 2 wherein the dashed line pattern is the radiation pattern from the error horn 16A and the solid line radiation pattern is the radiation pattern from the error horn 16B. Since these error horn radiation patterns crossover at the null between the first and second lobes, this results in the detection of the null which has a rapidly increasing amplitude on either side thereof when these two radiation patterns are combined. This rapidly increasing amplitude permits the precise location of the null position thus facilitating the accurate determination of the azimuth and elevation error signals. However, when the error horn spacing is fixed as shown in FIG. 2 so that the error horn's secondary pattern crossover is at the first null, the sum horn aperture size is such that a large spillover past the subreflector is incurred. This spillover is shown, as noted above, in FIG. 2 wherein the lined area on the periphery of the sum horn radiation pattern constitutes the radiation spillover. If the sum horn diameter is increased to reduce this spillover, the error horn spacing must also be increased, resulting in a crossover on the first sidelobe as shown in FIG. 3 where the dashed line and solid line patterns again represent the radiation patterns from the error horns 16A and 16B, respectively. Such a first sidelobe crossover clearly results in low amplitude signals on either side of the detected null.